POAG and tinnitus may be related via an underlying mechanism. Photo: Mark Paton on Unsplash. Click image to enlarge.

Previous studies have found an association between glaucoma and tinnitus, but one has reported that a history of tinnitus was linked with thicker retinal fiber layer. To expand on this, researchers investigated the relationship between pre-existing primary open angle glaucoma (POAG) and the condition and highlighted a significant association. 

Medical records of over two million patients were analyzed. The researchers found that 3.9% of subjects overall had POAG prior to tinnitus: 4.7% among the glaucoma cases and 3.7% among the control group. Regression analysis showed those with tinnitus were 30% more likely to have pre-existing POAG.

The underlying mechanism relating glaucoma and tinnitus is not exactly clear. Vascular dysregulation is one possible common pathway, according to the authors.

“Although POAG is multifactorial, and a clear pathophysiology is not established, it has been suggested that the glaucomatous ganglion cell damage is caused, at least in part by chronic impairment of blood supply to the optic nerve head,” they explained. “There is evidence that formation of nitric oxide by the endothelium and the neuronal isoforms of the enzyme nitric oxide synthase maintains the vasodilator tone in the optic nerve head.” 

Among patients with POAG, endothelial dysfunction in the eye combined with defective nitric oxide synthase can alter this balance, causing reduction of nitric oxide production and an increase in reactive oxygen species and endothelin-1 biosynthesis. Increased endothelin-1 activity, a vasoconstrictor, may lead to the pathologic features of optic neuropathies such as POAG.

Nitric oxide is also shown to be reduced in the jugular vein of tinnitus patients, which may lead to disruption of the microcirculation in the ear. The authors suggest that this theory, along with understanding the underlying mechanisms between POAG and tinnitus, should be further investigated.